In recent years, LTE (Long Term Evolution) that is one of high-speed data communication specifications is widely spread in 3GPP (Third Generation Partnership Project). A radio communication device such as a mobile device that employs LTE performs a discrete Fourier transform (DFT) process when transmitting data. More specifically, when performing the DFT process, the radio communication device multiplies a twiddle factor by a time-domain signal that is an input signal and sums the multiplication results to converts the time-domain signal into a frequency-domain signal.
However, in the radio communication device that employs LTE, the more the size of DFT in the DFT process increases, the more a calculation amount accompanied with the DFT process increases. In other words, a multiplication to be executed when performing the DFT process is a complex multiplication, and one complex multiplication includes four real number multiplications. For this reason, the more the size of DFT in the DFT process increases, the more the number of complex multiplications increases. Furthermore, the number of real number multiplications increases to the quadruple of the number of complex multiplications.
Therefore, a method in which a complex multiplication is omitted has been proposed in order to reduce a calculation amount accompanied with the DFT process. In other words, in the method, twiddle factors placed on the circumference of a unit circle on a complex plane are previously stored in a twiddle factor table in association with the phases of the twiddle factors. Moreover, in the method, the phase of an arbitrary twiddle factor stored in the twiddle factor table is added to the phase of an input signal, and a twiddle factor corresponding to the added phase is acquired from the twiddle factor table to sum the acquired twiddle factor. As a result, because a calculation result of the DFT process is obtained only by summing twiddle factors, a complex division can be omitted and thus a calculation amount accompanied with the DFT process can be reduced.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-060433
The conventional method has a problem in that the scale of a radio communication device increases when the amplitude of an input signal is not constant.
More specifically, when a signal of which the amplitude is 1 (constant value) is an input signal as with a signal modulated by QPSK (Quadrature Phase Shift Keying), for example, it is only sufficient that the radio communication device has one twiddle factor table that stores therein a twiddle factor of which the amplitude is “1”. However, when a signal that has three kinds of amplitudes is an input signal as with a signal modulated by 16QAM (Quadrature Amplitude Modulation), for example, the radio communication device has three twiddle factor tables that store therein twiddle factors according to the three amplitudes. As described above, because the radio communication device has a plurality of twiddle factor tables that respectively store therein twiddle factors according to amplitudes when the amplitude of an input signal is not constant, its circuit scale increases.